Heat transfer surface which promotes nucleate ebullition



July 28, 1970 R. L. WEBB 3,521,708

HEAT TRANSFER SURFACE WHICH PROMOTES NUCLEATE EBULLITION Filed Oct. 50, 1968 INVEN'TOR.

RALPH L. WEBB BY Z - ATTORNEY United States Patent O 3,521,708 HEAT TRANSFER SURFACE WHICH PROMOTES NUCLEATE EBULLITION Ralph L. Webb, Minneapolis, Minn., assignor to The Trane Company, La Crosse, Wis., a corporation of Wisconsin Filed Oct. 30, 1968, Ser. No. 771,873 Int. Cl. F28f 13/00 US. Cl. 165-486 15 Claims ABSTRACT OF THE DISCLOSURE A heat transfer surface composed of a base member, fins and an insert between the fins improves heat transfer. A gap is provided between the insert and the fins sufiicient to initiate and sustain nucleate boiling of a given liquid contacting said fins. The surface is easily adaptable from conventional structures. Materials, variations in structure, and alternatives are disclosed.

BACKGROUND OF THE INVENTION Field of the invention A highly effective way of transferring heat from a heated wall to a fluid contacting it is through the mechanism of nucleate boiling. According to the most commonly accepted theory of nucleate boiling, irregularities or cavities in the heat transfer surface known as nucleation sites trap minute amounts of vapor which form the nucleus of a bubble. The bubbles grow and detach from the surface as the liquid on the surface if heated above its saturation temperature. Incipient boiling or initial significant bubble formation requires that the nucleation sites be covered by a certain thickness of superheated liquid. As the bubbles rise in continuous columns from nucleation sites, they interrupt the boundary layer of superheated liquid and carry superheated liquid away from the hot wall surface. It is believed that the greater part of vapor formation in nucleate boiling occurs as superheated liquid evaporates into the liquid-vapor interface of rising bubbles. Moreover, the agitation of the liquid by rapid bubble departures increases the rate of heat transfer to the liquid by forced convection.

These advantageous heat transfer eifects associated with the bubble columns indicate that surface heat transfer should be high in the locality of the bubble emission sites. It can then be assumed, as a natural corollary, that the heat transfer rate, and especially the boiling heat transfer rate, will increase in direct proportion to the number of active bubble column sites. The experimentation resulting in this invention, as well as the work of others, have established that this is indeed the case. See for example, H. M. Kurihari and J. E. Myers, The Effects of Superheat and Surface Roughness on Boiling Coefiicients, American Institute of Chemical Engineers Journal, vol. 6, No. 1, pp. 83-91 (1960). Thus, for a given temperature differential (AT) between the temperature of a hot wall (T and the saturation temperature of a liquid (T in contact therewith, the boiling heat transfer coefficient will vary with the area density of nucleation sites. It is therefore highly desirable from a performance standpoint to treat or condition a heat transfer surface in a manner which causes greater density of bubble columns for a particular value of AT.

Description of the prior art It is known that artificial nucleation sites can be created in a surface by techniques see, I. A. Clark, Theory and Fundamental Research in Heat Transfer, p. 64, Pergamon Press (1963) and U.S. Pat. No. 3,301,314. For

other surfaces which promote nucleate boiling see U.S. Pat. No. 3,326,283. Surfaces which have been roughened to produce a large number of discrete pits, scratches or cavities of microscopic size which act as good vapor traps have been found to be good nucleate boiling surfaces. The problem in attempting to utilize the mechanism of nucleate boiling on a commercial basis lies in arriving at a particular surface geometry which has a large population density of nucleation sites to 200 per square inch), and which can be consistently and economically reproduced in relatively large quantities.

BRIEF SUMMARY OF THE INVENTION This invention provides a commercially useful, inexpensive heat exchanger which can be simply manufactured from existing construction materials. The invention comprises a heat conductive base member and a plurality of spaced, heat conductive fins attached to and extending from the base member. The fins form grooves in which an insert is placed. The insert substantially bridges at least a portion of this groove. There is a gap between the insert and the side walls of the fins which is suflicient to sustain nucleate boiling of a given liquid which contacts the fins. The size of the gap is dependent upon the physical characteristics of the liquid to be boiled. The preferred embodiment of this invention comprises a tubular base member which has radially extending fins formed in a helical configuration about the tubular base member. The insert is preferably either a continuous length of heatconductive wire or a continuous length of nonheat-conduc tive insert material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevation view of a finned tube illustrating the placement of an insert in the grooves formed by the fins on a tube to provide the nucleate boiling surface of the instant invention.

FIG. 2, is a cross sectional view of the two adjacent fins, the groove formed thereby, and the insert in the groove.

FIG. 3 is a cross sectional view similar to that of FIG. 2 showing a polymeric insert in the grooves.

FIG. 4 is a cross sectional view similar to FIG. 2, showing an elongated wire insert in the groove.

FIG. 5 is a diagrammatic view of a refrigeration sys tem including an evaporator in which the improved nucleate boiling surface can be employed.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates an application of the present invention to a commercially available finned tube. A plurality of spaced apart fins I extend from a heat conductive base member 2, here shown as a tube. The fins are shown as connected in a continuous helical pattern. The fins 1 can be made from a separate material from the base member and attached to the outer surface of the base member, or they can be machined from the base member to be integral therewith. If the fins were formed separately, they would be ring like members which would be placed upon and attached to the base member 2. An insert 3 is laid in the grooves 5 which are formed by the walls of the fins 1. Insert 3 is attached to base member 2 at point 4 by conventional means for bonding the materials from which the insert and fins are manufactured. The insert 3 in FIG. 1 is shown as a continuous length of insert material wound through the helically formed grooves of the fin structure. As will be described later it is not necessary that the insert 3 be of circular cross section. It can be of rectangular, oval or any other shape Which does not depart from the inventive concepts.

Referring now to FIG. 2, there is illustrated a cross sectional view of a portion of a heat conductive base member similar to that of FIG. 1. Base member 6 has fins 7 attached to or formed thereon. Insert 8 is positioned in groove 9 to fill almost the entire groove in a longitudinal direction. Gaps, designated a in the figure, must be present in order to initiate nucleate boiling. The size of the gap depends almost entirely upon the physical characteristics of the boiling liquid which is contacting the fins 7. For example, where Refrigerant 11 (trichloromonofiuorornethane) is the boiling liquid to which heat is being transferred, the preferred gap size, a, can range from 0.0005 to 0.002 inch. Of course, the distance between the fins 7 and the size of the insert 8 will exert limitations upon the size of the gap a which can be employed.

FIGS. 3 and 4 illustrate other embodiments of the present invention. The views in FIGS. 3 and 4 are similar to that shown in FIG. 2, i.e., a base member 6 has fins 7 attached or integrally formed therefrom. An insert 8 is positioned in the groove formed by the fins 7. Although FIGS. 1 through 4 illustrate a preferred embodiment of my invention it is to be understood that my invention is not to be limited to a base member which comprises a tubular surface. The surface to which the fins and inserts of the present invention are applied can be flat surface, a surface of rectangular cross section, or any other surface which is adaptable for heat transfer.

FIG. illustrates diagrammatically a standard compression refrigerator system with a shell and tube evaporator in which the fin insert tube surface can be used. Evaporator 20 is connected in a refrigeration circuit including compressor 22, condenser 24, and flow regulating valve 26. Either a reciprocating or centrifugal type compressor can be employed; a centrifugal compressor 22 has been shown for illustrative purposes. Evaporator 20 has a shell 21, headers 23 and 25, and closely spaced tubes for conducting fluid to be cooled from inlet header 23 to outlet header 25. Water or other fluid to be cooled flows from inlet 28 through tubing 30 and is discharged through outlet 32. Refrigerant liquid from condenser 24 is expanded into shell 21 as it flows from control valve 26. The refrigerant which enters evaporator 20 is a mixture of liquid and vapor. The liquid is evaporated as the refrigerant flows through shell 21 in contact with the outside of tubing 30. Heat transfer to the refrigerant thus takes place by the combined modes of forced conduction and nucleate boiling, thus making it more difiicult to predict the total increase in heat flux to be realized by improving the nucleate boiling performance of finned tubing 30. It has been demonstrated that a significant increase in total heat flux is achieved by utilizing the fin insert surface under such conditions. The net increase in heat flux closely approximates the direct addition of the pool boiling and forced conduction heat fluxes for a particular surface and fluid.

The mechanism which operates to improve the performance of the fin-insert surface is one of conventional nucleate boiling. The narrow gap formed between the fin wall and the insert material creates a continuous array of nucleation sites. Thus the fin-insert surface provides a simple inexpensive article which promotes nucleate boiling.

Although the heat transfer surface of this invention has been described above in conjunction with a parallel wall fin structure, the invention is not limited by that description. The groove formed by the fins may be V-shaped, or any other similar configuration as long as a gap remains between the fin wall and insert which is suflicient to initiate and sustain nucleate boiling at a low AT. The base member of the invention can be tubular, fiat or other heat transfer surface adaptable to the inventive concept. The fins can be integral with the base member or can be separately formed and placed in heat transfer relationship with the base member. The insert can be placed in the groove in segments or can be continuous, for example, in a helically formed, finned, tubular heat transfer structure. It is desirable, in fact, preferable to have the insert adjacent all fin walls of the heat transfer structure, as the most eflicient form of this invention will then be formed. As previously noted, the base member and fins need not be made of the same material. The insert can be composed of a material having a low heat conductivity. A slight performance increase is obtained when heat conductive material is utilized; however, the cost advantage is lost.

This invention affords high nucleate boiling performance by simple modification of ordinary finned surfaces, using low cost insert materials. This alteration does not involve special machining or metal forming techniques commonly associated with formation of artificial nucleation sites. The utilitarian value of the invention is apparent.

What is claimed is:

1. A heat exchanger comprising:

(A) a heat conductive base member,

(B) a plurality of spaced, heat conductive fins attached to and extending from said base member, said fins having grooves therebetween,

(C) an insert in said grooves substantially bridging at least a portion of said groove, there being a gap between said insert and said fins sufficient to initiate and sustain nucleate boiling of a given liquid contacting said fins, the gap between said insert and the nearest of said fins being in the range of from 0.0005 to 0.002 inch.

2. The heat exchanger of claim '1 wherein the fins form a groove having a lower portion of substantially the same width as the upper portion of said groove.

3. The heat exchanger of claim 1 wherein the fins form a groove having a lower portion of substantially lesser width than the upper portion of said groove.

4. The heat exchanger of claim 1 wherein the said base member is tubular.

5. The heat exchanger of claim 4 wherein the said fins are integral with said base member.

6. The heat exchanger of claim 5 wherein the said fins are helically formed on said base member.

7. The heat exchanger of claim 6 wherein the said insert is continuous throughout the groove formed by said fins.

8. The heat exchanger of claim 7 wherein the said insert is a wire of substantially circular cross section.

9. The heat exchanger of claim 1 wherein said base member, said fins and said insert are composed of the same material.

10. The heat exchanger of claim 9 wherein the said material is a heat conductive metal.

11. The heat exchanger of claim 1 wherein said base member is composed of a first heat conductive material, said fins are composed of a second heat conductive material, and said insert is composed of a third material.

12. The heat exchanger of claim 11 wherein the first and second heat conductive materials are the same.

13. The heat exchanger of claim 12 wherein said third material is polymeric.

14. The heat exchanger of claim 13 wherein said third material is a polytetrafluorethylene.

15. The heat exchanger of claim 1 wherein said insert substantially bridges the entire height of said groove.

References Cited UNITED STATES PATENTS 2,721,729 10/1955 Van Riper -64 3,045,138 7/1962 Pohl 165l84 3,326,283 6/1967 Ware 165184 3,454,081 7/1969 Kun et al. 165-133 ROBERT A. OLEARY, Primary Examiner C. SUKALO, Assistant Examiner U.S. Cl. X.R. 

